This invention relates to a machining control system for controlling machining state(s) of machine tools and, more particularly, to a machining control system made up of numerical controllers (NCs) which has functions of automatically determining machining conditions or automatic programming systems, or machining control systems made up of the combination of programming devices such as CAMs with numerical controllers.
In machining, it is not easy to determine the machining conditions such that the efficiency is high, the precision is high and no abnormality occurs because a large number of factors such as tools, workpieces to be machined (hereinafter referred to as workpieces), machines and coolants are complicatedly related to each other. For that reason, up to now, the machining conditions have been automatically determined by the following main two approaches.
One of those approaches is directed to a machining condition determining system disclosed in Japanese Patent Publication No. 7-41516 (refer to FIG. 13), Japanese Patent Laid-Open No. 2-218538, Japanese Patent Laid-Open No. 10-86039, Japanese Patent Laid-Open No. 3-43132, and the like. This system mathematically represents and retains the knowledge of skilled men for determining the machining condition in accordance with various factors such as the tool and the workpiece which influence the machining so as to automatically determine the machining conditions. Further, it is general to provide a mechanism for additionally learning the knowledge for determining the machining condition. In this approach, the knowledge is normally acquired by the learning in an off-line manner, and this approach is mainly applied to CAD/CAM, an automatically programming system, or the like. In the following description, the approach of this type is called xe2x80x9cfirst approachxe2x80x9d.
Another approach is directed to an adaptive control system disclosed in Japanese Patent Laid-Open No. 62-292347 (refer to FIG. 14), Japanese Patent Laid-Open No. 2-218538, Japanese Patent Laid-Open No. 6-8106, and the like. In this system, the machining states are monitored on the basis of signals obtained by sensors or the like and the machining conditions are changed in such a manner that an objective function is optimized while keeping given constraints, to thereby control the machining in an appropriate state. This approach changes the machining conditions at a real time in accordance with the actual machining states, and is mainly applied to the numerical controller or the like. In the following description, the approach of this type is called xe2x80x9csecond approachxe2x80x9d.
Those conventional systems will be complementarily described in more detail. FIG. 13 is a block diagram showing the structure of conventional system using the first approach (disclosed in Japanese Patent Publication No. 7-41516) among the above conventional approaches. In the figure, reference numerals 5 and 11 denote a spindle motor and a grinding motor which are to be controlled, respectively. Reference numeral 14 denotes a first RAM in which data related to the shape and material of the spindle (workpiece), etc., and data related to the tool and the machining method are stored. Reference numeral 15 denotes a second RAM for storing standard machining data which are set therein, 16 is an automatic machining condition setting circuit for automatically setting the machining condition on the basis of the data stored in the first and second RAMs, 18 is a CRT for displaying the machining condition which is automatically set, and 12 is a keyboard used in the case where an operator judges that the machining condition displayed on the CRT 18 is improper and corrects the machining condition. Further, in the figure, a portion xe2x80x9caxe2x80x9d surrounded by a phantom line is means for storing a corrected rate as the coefficient every time when the operator corrects the machining condition and automatically correcting the machining condition by using the data in the next machining.
The operation of the conventional system shown in FIG. 13 will be described in brief. In the automatic machining condition setting circuit 16, the machining condition is automatically determined on the basis of the data retained in the first and second RAMs, and if a correction is necessary, the operator conducts the correction. In addition, when the correction has been conducted, the rate of the correction is retained as the coefficient, and the coefficient is used to conduct the automatic correction in the next machining.
Further, FIG. 14 is a block diagram showing the structure of a conventional system using the second approach (disclosed in Japanese Patent Laid-Open No. 62-292347) among the above conventional approaches. In the figure, reference numeral 40 denotes machining load detecting means for detecting a machining load, 41 is standard machining load deriving means for sampling an output of the machining load detecting means 40 at the time of a model machining using a reference tool to derive the standard machining load on the basis of the sampled output, and 42 is machining condition setting means for setting the machining condition at the time of actual machining using a tool similar to the reference tool. Reference numeral 43 denotes target load calculating means for calculating a target load on the basis of the standard machining load derived by the standard machining load deriving means 41 and the machining condition at the time of actual machining which is set by the machining condition setting means 42, and 44 is feed rate control means for increasing or decreasing a feed rate such that the machining load detected by the machining load detecting means 40 at the time of actual machining becomes the target load calculated by the target load calculating means 43.
The operation of the conventional system shown in FIG. 14 will be described below. The model machining is conducted on only the reference tool, and the machining load at the time of the model machining is detected by the machining load detecting means 40, and the standard machining load deriving means 41 derives the standard machining load on the basis of the machining load thus detected. In the actual machining time, the target load calculating means 43 is actuated and calculates the target load on the basis of the machining condition at the time of actual machining which is set by the machining condition setting means 42 and the reference machining load derived at the time of model machining. The feed rate control means 44 compares the target load thus calculated with the actual machining load detected by the machining load detecting means 40 to change the feed rate such that the detected actual load approaches the calculated target load.
The conventional first approach is made provided that rules or models which determines the machining condition on the basis of the factors that basically influence the machining is identical with the actual machining environment and a fluctuation of the actual machining environment is sufficiently small. However, the above provision is hardly satisfied for the following reasons.
In general, in machining, there are many indefinite elements because the phenomenon is physically unclear, the phenomenon occurs only stochastically, much labor is required for making models, and the like. For example, in cast material, it has been known that variation of hardness by several tens of % cannot be prevented due to the metal composition, heat flow conditions, and the like at the time of manufacturing a casting. Similarly, in the tool, it has been known that the sharpness of the tool varies by several tens of % depending on the metal composition, coating state, grinding precision, and the like, even if a tool of the same model number is repeatedly used. In workpieces or tools which are manufactured at the same time, that is, of the same lot, the variation is somewhat improved, but slight variation cannot be prevented. There are many elements that are difficult to estimate such as not only the variation of the workpiece and the tool but also chip jamming, chipping, the state of coolant, or the like, which suddenly change during machining.
Therefore, in the conventional first approach, in order to prevent an abnormality from occurring even if the unexpected phenomena occur, a sufficiently safe machining condition must be set, increasing loss, as a result of which the efficiency is lowered.
On the other hand, the conventional second approach may be insufficient from the viewpoint of efficiency and reliability although it can cope with variations of the machining environment to some degree. That is, because a delay of detection and control always exists in adaptive control, there is the possibility that it will take a long time to reach the optimum state or that an abnormality may occur until reaching the optimum state, depending on the initial machining condition. In particular, this leads to problems in high speed machining which has been increasing in recent years. For example, in a drilling process using a drill, there is a case where it does not take 0.1 seconds to drill one hole, which is not sufficiently long as compared with a normal delay (several tens msec.) of control in the adaptive control. That is, there is the possibility that the state does not reach the optimum state. As described above, since a delay occurs in the adaptive control, if the initial machining conditions are improper, there arises a problem from the viewpoint of reliability. Further, there arises such a problem in that it is difficult to estimate the tact time as a problem inherent to adaptive control. This is a reason why the second approach is not applied to a mass production line.
The above problems are caused by the separation of the determination of the machining conditions at the machining information determination stage from the adjustment of the machining conditions at the adaptive control stage. That is, the problems reside in that the former does not take the actual machining environment and the adaptive control manner into consideration, and the latter only faithfully keeps to the given condition (the machining conditions and the control parameters).
The present invention has been made in order to solve the above problem, and therefore an object of the present invention is to provide a machining control system which balancedly combines the determination of the machining conditions at a machining information determination stage with the adjustment of the machining conditions at an adaptive control stage, thereby being capable of keeping the optimum machining state and enhancing the efficiency and the reliability even in the case where an actual machining environment fluctuates.
With the above object(s) in view, the machining control system of the present invention comprises a machining data base for storing information necessary for determining initial machining conditions, machining information determining means for obtaining the initial machining conditions on the basis of the information stored in the machining data base and in accordance with adaptive control characteristics defined by adaptive control modes and adaptive control parameters and adaptive control means for controlling the machining in appropriate states by changing the machining conditions in accordance with the machining states observed during the machining with the initial machining condition as initial value of the machining conditions.
The machining data base may store therein information on tool characteristics, workpiece characteristics, relational expression of the machining states to the tool characteristics/the workpiece characteristics and the machining conditions, standard machining conditions and machining state target values.
In the case where there are a plurality of selectable adaptive control characteristics, the machining information determining means may select the adaptive control characteristics such that any one of a machining period of time, a shape error, an abnormality occurrence rate and a tool wear amount becomes minimum among the plurality of adaptive control characteristics, and determines the initial machining conditions suitable for the adaptive control characteristics selected.
The adaptive control means may analyze at least one of the tool characteristics and the workpiece characteristics on the basis of the machining states that are observed through the machining and the machining condition, and updates at least one of the informations of the tool characteristics and the workpiece characteristics which are retained in the machining data base.
The machining data base may be structured by characteristics inherent to the tools and the workpieces now used, characteristics common to the tools and the workpieces of the same lots as those of the tools and the workpieces now used, and characteristics common to all of the tools and the workpieces of the same kinds of the tool and the workpiece now used.
In the case where the machining is conducted first after the tool has been exchanged, the machining information determining means may obtain the machining conditions such that the expected machining states do not exceed machining state target values taking the characteristics common to the tools of the same lot or the same kind and variations in the characteristics of the tools into consideration which are stored in the machining data base, and the machining conditions are determined as the initial machining condition.
In the case where the machining is conducted first after the workpiece has been exchanged, the machining information determining means may obtain the machining conditions such that the expected machining states do not exceed machining state target values taking the characteristics common to the workpieces of the same lot or the same kind and a variation in the characteristics of the workpieces into consideration which are stored in the machining data base, and the machining conditions are determined as the initial machining condition.
The adaptive control means may judge that the abnormality occurs if the tool characteristics and the workpiece characteristics which are referred to in determination of the machining conditions in the machining information determining means are largely different from the tool characteristics and the workpiece characteristics which are analyzed in the adaptive control means, respectively, and updates at least one of the information of the tool characteristics and the workpiece characteristics with respect to the machining data base, and the machining information determining means re-determines the adaptive control characteristics and the machining conditions to prevent the abnormality.